Fuel purge control

ABSTRACT

A method for controlling delivery of fuel from a vapor collection device to the combustion chamber of an internal combustion engine to thereby purge fuel that has accumulated in the vapor collection device by means of a purge flow passing from the vapor collection device to the engine, the purge flow rate being varied by means of a flow control valve located between the vapor collection device and the engine, the method including determining a minimum valve signal value and a maximum valve signal value as a function of the engine load end engine, the method including determining a minimum and maximum extent of valve signal values for controlling the opening of the valve, selecting either the minimum or maximum valve signal value or an interpolation between the maximum and minimum values in dependence on the engine operating conditions to optimize the amount of fuel purged from the vapor collection device to the engine under varying engine operating conditions. In an internal combustion engine having a primary fuel source, and a system for delivering fuel vapor produced in the fuel system of the internal combustion engine to at least one combustion chamber thereof, a method of determining the amount of fuel vapor being purged during closed loop operation of the engine, including: determining the amount of fuel being provided to the engine by the primary fuel source; and comparing this value to a predetermined estimate of required total fuelling level.

This invention relates to the control of the purging of fuel from thefuel vapour collection device for an internal combustion engine.

The current emission regulations in many countries require theevaporative emissions from the fuel supply system of the internalcombustion engines of motor vehicles to be controlled to therebyeliminate or substantially reduce the amount of fuel released into theatmosphere by such vapours. Accordingly, it is normal practice to fit afuel vapour collection device to the vehicle to adsorb evaporativeemissions from the fuel supply system under all conditions that thevehicle experiences. This fuel vapour collection device is usually ofthe activated carbon type and is commonly referred to as the “carboncanister”. Such a fuel vapour collection device operates on theprinciple of physical adsorption of fuel vapour into the activatedcarbon.

The fuel vapour collection device has a limited capacity for storingfuel and must therefore be purged to some extent of its contents in thecourse of vehicle operation. The accumulated fuel is normally purgedinto the intake manifold of the engine by air drawn through the fuelvapour collection device, the purged fuel being subsequently combustedby the engine. The amount of fuel vapour being purged from the fuelvapour collection device can however vary significantly for any givenpurge air flow rate generally depending on saturation level in the fuelvapour collection device. As the amount of purged fuel is not normallymeasured in systems not having an air/fuel ratio feedback mechanism(commonly known as open loop systems), the engine control system cannotcompensate for the increased fuelling rate to the engine. This can causean increase in the engine torque which results in a higher engine speedat idle or an increase in the vehicle speed off idle. Under severeconditions, the engine operation can become unstable because the actualair fuel ratio within the engine cylinders is markedly different fromthe air fuel ratio mapped by the engine control system.

One proposal to deal with this problem is described in the applicant'sU.S. Pat. No. 5,245,974. This document shows an internal combustionengine installation having a fuel vapour collection device for removingthe fuel vapour from the evaporative emissions generated within the fuelsupply system. The engine includes a fuel injection system with an aircompressor supplying compressed air to the fuel injection system. Thefuel vapour collection device is periodically purged of accumulated fuelby drawing air through the fuel vapour collection device using the aircompressor. The air compressor then supplies the air which now carriesthe fuel to the fuel injection system where the air is subsequentlyinjected into the combustion chambers of the engine resulting incombustion of the purged fuel. Although the stratification within thecylinder will remain largely unaltered by the addition of the purgedfuel through the injector, this patent does not particularly address theproblem of lack of knowledge of the amount of fuel being supplied fromthe fuel vapour collection device.

It would be advantageous to provide a system which can control the airflow rate through the fuel vapour collection device to optimise theamount of fuel that is purged from the fuel vapour collection devicewithout jeopardising engine operation.

With this in mind, it is an object of the present invention to providean improved method and control system for controlling the air flow ratethrough an fuel vapour collection device for an internal combustionengine.

According to one aspect of the present invention, there is provided amethod for controlling delivery of fuel from a vapour collection deviceto the combustion chamber of an internal combustion engine to therebypurge fuel that has accumulated in the vapour collection device by meansof a purge flow passing from the vapour collection device to the engine,the purge flow rate being varied by means of a flow control valvelocated between the vapour collection device and the engine, the methodincluding determining a minimum valve signal value and a maximum valvesignal value as a function of the engine load and engine speed tothereby respectively define the minimum and maximum extent of valvesignal values for controlling the opening of the valve, selecting eitherthe minimum or maximum valve signal value or an interpolation betweenthe maximum and minimum values in dependence on the engine operatingconditions to optimise the amount of fuel purged from the vapourcollection device to the engine under varying engine operatingconditions.

This method enables at least substantially continuous purging of thefuel vapour collection device and enables the amount of fuel purged fromthe fuel vapour collection device to the engine to be optimised forvarying operating conditions of the engine.

The fuel vapour collection device may be in communication with an intakemanifold of the engine and the method can therefore control the amountof fuel purged into the intake manifold. The pressure difference betweenthe fuel vapour collection device and the intake manifold may besufficient to enable air to be drawn through the fuel vapour collectiondevice to the intake manifold. The present method may however also beused in other arrangements, for example when there is purging through anair compressor as described in U.S. Pat. No. 5,245,974 referred toabove.

The method may be implemented by a variable valve for controlling theair flow rate from the fuel vapour collection device and a control meansfor controlling the valve as a function of the engine operatingconditions. The control means may be in the form of an electroniccontrol unit (ECU) for providing the valve with the required valvesignal values for controlling the progressive opening anrd closing ofthe valve. A given valve signal may correspond to a given valveposition.

The ECU may include at least two “look-up” maps for mapping the valvesignal values for controlling the valve as a function of engineoperating conditions. Each look-up map may provide valve signal valuesagainst the coordinates of fuel per cycle (FPC) and engine speed (RPM).One of the maps may be a “minimum” map which maps the valve signalvalues when the amount of purge air flow to the engine is required to beat a minimum level. This situation can for example arise when air purgedfrom the vapour collection device is very rich in fuel vapour. Anothermap may be a “maximum” map mapping the valve signal values when thepurge air flow to the engine can be maximised. This takes into accountsituations where the air fuel ratio of the purge air is relatively lowand the engine is operating at medium to high loads.

The minimum and maximum maps may therefore respectively define theminimum and maximum range of valve signal values for controlling theopening of the valve and therefore the purge air flow rate, the openingof the valve progressively increasing with increasing valve positionvalues. The valve signal value may be obtained from either of these mapsor from an interpolation between these maps in dependence on the engineoperating conditions. The interpolation amount may be provided by anadaption value. The adaption value may be provided by an arbitrary valuesystem which assigns a proportion of each of the minimum and maximumvalues to create a total value for valve position determinationaccording to given condition. This adaption value may lie within therange of 0.0 to 1.0, with the 0.0 value corresponding to the minimum mapand the 1.0 value corresponding to the maximum map.

An adaption value look-up map may map the adaption values as a functionof the engine coolant temperature. Upon first starting up the engine,the water temperature may be measured and an initial adaption valueobtained from the adaption value map. This ensures that if the engine isstarting hot, the adaption value may be relatively low to restrict theflow of purged air. Under hot starting conditions, relatively largeamounts of fuel vapour may have been generated within the fuel tankwhich is then adsorbed within the fuel vapour collection device. Thevalve position following a hot start can therefore prevent excessivefuel being purged at that time.

Advantageously, the present method may be applied to engine controlsystems which generally operate under open loop control (that is,without exhaust air/fuel ratio feedback), and which are provided withengine speed feedback at idle to control idle operation (known as closedloop speed control). In such systems, the ECU monitors engine speed atidle and alters fuelling (whether by direct control of the fuelling rateor by air flow rate control) to maintain engine speed at the desiredidling speed.

In a preferred embodiment, the primary fuel supply to the engine isprovided by one or more fuel injectors so that supply of fuel other thanfrom the fuel vapour collection device can be accurately controlled.This can be done via manifold or direction injection.

According to the present method, the adaption value may be periodicallyvaried with changing operating conditions of the engine. When the engineis at idle, the adaption value may be varied by comparing the actualidle fuelling level to a preset target fuelling level. This targetfuelling level may be a mapped value provided by the ECU. The enginetypically operates under closed loop engine speed control at idle. Underclosed loop operation, feedback on the engine speed is supplied to theECU which then seeks to maintain a constant engine speed by altering thefuelling level. This idle fuelling level is compared against the presettarget fuelling level. The target fuelling level would typically bebelow the normal fuelling level of the engine at idle where no fuel isprovided through the fuel vapour collection device but above a fuellinglevel that would cause combustion instability or loss of engine controlby the ECU.

If the idle fuelling level is above the target fuelling level, then theadaption value may be incremented by a specified amount. This would forexample take into account the situation where there was little fuelcoming from the fuel vapour collection device so that the idle fuellinglevel would be high because it was not being supplemented by fuel fromthe fuel vapour collection device. As such, where the actual idlefuelling level is above the target value, the valve controlling thepurge air flow rate could be opened further by a small amount. This iseffected by incrementing the adaption value.

On the other hand, if the idle fuelling level is below the targetfuelling level, then the adaption value may be decremented by a presetamount. This for example takes into account the situation where a lot offuel was being purged from the fuel vapour collection device to theengine. This would result in the actual idle fuelling level being low asa result of the reduction in the fuelling level being initiated by theECU under closed loop speed control to keep the engine idle speed at apredetermined level. If the fuelling level through the primary fuelsupply was low with a significant amount of fuel being purged from thefuel vapour collection device, then the combustion within the combustionchambers may become unstable. In this case, the adaption valve may bedecremented to reduce the flow through the fuel vapour collection deviceand thus the amount of fuel being purged from the fuel vapour collectiondevice into the combustion chamber. To help improve combustionstability, an air flow offset may be added when the adaption value isdecremented to increase the bulk air flow to the engine. Furthermore, anair flow offset may be removed if the adaption value is incremented. Theairflow offset may simply be the addition of a specified airflow to thebulk manifold airflow by adjustment of an electronically controllableairflow device such as a DAR-valve device as described in theApplicant's U.S. Pat. No. 5,251,597.

Idle fuelling level may be compared to a limit value to check whether itis too low (ie in the case where too small a proportion of total fuelsupplied is being delivered through the injector). If this condition isextreme, such that engine instability is likely due to a high proportionof the fuel being supplied from the vapour collection device, then theadaption value may be set (preferably immediately) to 0.0 to promptlyreduce the possibility of combustion instability within the engine. Itmay also be preferable to add the above noted air flow offset when theadaption value is set to zero if so required.

In certain systems, the engine operates in open loop mode when off-idle,and there is no mechanism to enable measurement of fuel being deliveredfrom the fuel vapour collection device. In this case, it would bepossible to assume that conditions under which the engine is operatingdo not change, and simply operate the fuel vapour collection device atthe adaption rate set by the previous idle adaption value setting.However, as operating conditions may change over time, there is anincreasing uncertainty as to whether the previous setting of theadaption value at idle is appropriate for the present conditions.

The adaption value may therefore be progressively reduced when theengine is off idle and in a running mode to compensate for theincreasing uncertainty of the fuel concentration in the purged air asthe period since the last determination of the purge rate increases. Tothis end, the adaption rate may be periodically decremented by aspecified amount during an off-idle period of engine operation. Forexample, if the engine was cold on start-up and the adaption value setat a relatively high value, and the vehicle driven by the engineoperated for a period of time, then the temperature of the fuel withinthe fuel tank could increase substantially leading to increasedevaporative emissions and an increase in the charge of the fuel vapourcollection device. Therefore, if the purge rate was not reduced, thenthe purge rate may be inappropriate at light loads causing poor runningof the engine.

The adaption value may therefore be determined and varied during thefollowing operating stages of the engine:

a) at engine start-up,

b) in idle mode of the engine, and

c) in running mode at specified time intervals since the last adaptionvalue check.

According to another aspect of the present invention, there is providedin an internal combustion engine having a primary fuel source, and asystem for delivering fuel vapour produced in the fuel system of theinternal combustion engine to at least one combustion chamber thereof, amethod of determining the amount of fuel vapour being purged duringclosed loop operation of the engine, including:

determining the amount of fuel being provided to the engine by theprimary fuel source; and

comparing this value to a predetermined estimate of required totalfuelling level.

A compensation factor may be applied to the predetermined estimate ofrequired total fuelling level to compensate for additional loading ofthe engine. The additional loading may for example be applied by an airconditioning unit or other known specific energy drain on the engine,and a specific compensation factor is applied in relation to each knownenergy drain on determination that the particular energy drain has beenapplied.

Closed loop operation may occur whilst the engine is in idle mode. Thepredetermined estimate of required total fuelling level may be providedby a pre-calibrated look-up map in an electronic control unit of theengine. Alternatively, or in addition, the predetermined estimate ofrequired total fuelling level for a given set of operating conditionsmay be provided by operating the engine at that given set of conditionswith a zero level of purge.

To provide a better understanding of the present invention, an exemplarycontrol strategy according to the present invention will be describedwith respect to the accompanying drawings. It is however to beappreciated that the present invention is not restricted to theparticularities of the described control strategy and does not supersedethe generality of the preceding description.

In the drawings:

FIG. 1 is a flow chart showing the control strategy for determination ofthe valve control signal;

FIG. 2 is a flow chart showing the determination of the adaption valueat cranking of the engine;

FIG. 3 is a flow chart showing the determination of the adaption valueat the idle mode of the engine; and

FIG. 4 is a flow chart showing the determination of the adaption valueduring the running mode of the engine.

The control strategy according to an embodiment of the present inventionrequires a valve for controlling the air flow through the fuel vapourcollection device and an electronic control unit (ECU) for providingcontrol signals to the valve. The valve is typically in the form of anelectromagnetically actuated valve and is referred to herein as thepurge solenoid valve.

The ECU includes two look-up maps, each respectively plotting purgesolenoid signal values against the co-ordinates of engine fuelling leveland engine speeds. One of the look-up maps, is the “maximum” map whichprovides valve signal values corresponding to maximum purge air flow tothe engine for the given operating conditions of the engine. The othermap provides valve signal values for when the purge air flow ratethrough the fuel vapour collection device is required to be at aminimum. These look-up maps respectively define the minimum and maximumextent of the range of valve signal values for the purge solenoid valvefor given engine load and speed values. Valve signal values between thetwo maps can be obtained by means of an adaption value, thedetermination of which will be subsequently described. This adaptionvalue allows valve signal values to be interpolated between the twomaps. The adaption value lies between the range of 0.0 to 1.0 with the0.0 value corresponding to valve signal values from the minimum map andthe 1.0 value corresponding to valve signal values from the maximum map.

It should be noted that at higher levels of engine fuelling and enginespeed, the amount of purging through the fuel vapour collection deviceon the basis of the minimum map need not be significantly different fromthe maximum map. It is most important at low fuelling and engine speeds,where excess fuelling through the fuel vapour collection device is morelikely to impact on engine operation, that a greater differential existsbetween maximum and minimum maps to enable the engine control system todecrease the amount of air flowing through the fuel vapour collectiondevice by tending towards a lower adaption value corresponding to theminimum map.

Referring to FIG. 1, the purge solenoid control signal is determined asfollows. Firstly, a valve signal value 6 is respectively obtained fromthe maximum map at step 1 and a second valve signal value 7 obtainedfrom the minimum map at step 2, each map using the actual fuel per cycleto the engine (FPC) and engine speed as co-ordinates when obtainingtheir respective valve signal values. At step 3 the valve signal value 6from the maximum map is multiplied by an obtained adaption value 8. Atstep 4, the valve signal value 7 obtained from the minimum map ismultiplied by 1 minus the same adaption value 8. The results from steps3 and 4 are then added at step 5 provide the purge solenoid controlsignal 9.

The adaption value is obtained and varied under different engineoperating modes as follows. Referring initially to FIG. 2, the initialadaption value is obtained during cranking of the engine. The coolanttemperature at cranking of the engine is obtained by means of a coolanttemperature sensor at step 10. The ECU includes an adaption valuelook-up table which plots the adaption value against the coolanttemperature. Therefore, at step 11, the initial adaption value isobtained as a function of the coolant temperature at cranking. Thecontrol strategy therefore commences using this initial adaption value.

During engine idle, the adaption value can be varied according to thecontrol strategy as shown in FIG. 3. At step 20, the time since the lastvariation of the adaption value within the idle mode or the time sincethe entry to the idle mode is compared against a specified time. Ifthese noted times are less than the specified time, then the adaptionvalue remains unchanged as shown in step 23. However, if the noted timesare greater than the specified time, then the adaption value is variedas follows. At step 21, the closed loop idle fuelling level isdetermined. At step 22, this idle fuelling level is compared against atarget value provided by the ECU. If the idle fuelling level is greaterthan the target value, then the adaption value is incremented by aspecified amount at step 27. This incremented adaption value then formsthe adaption value for the control strategy. However, if the idlefuelling level is less than the target value, then the idle fuellinglevel is compared against a limit value also provided by the ECU at step24. If the idle fuelling level is greater than the limit value, then theadaption value is decremented by a specified amount at steps 26.Furthermore, a specified airflow offset is added. If the idle fuellinglevel is less than this limit value, then the adaption value is set to0.0 and the specified airflow offset is added at step 25.

In one embodiment, the primary source of fuel to the combustion chamberof the engine is a metered fuel injection system. In this case theamount of fuel delivered by the primary source of fuel can be determinedby monitoring the fuel metered by the injection system. As such, theamount of fuel delivered to the combustion chamber from the vapourcollection device can be reasonably accurately determined.

Further arrangements may be provided to improve the combustion stabilityin the combustion chamber. One arrangement is the use of airflow offsetsindependent from the control of the air flow through the fuel vapourcollection device. For example, in steps 25 and 26, specified airflowoffsets may be added to increase the bulk air flow to the engine. Theincrease in the fresh airflow decreases the possibility of instabilityin the combustion chamber. It is also envisaged that a specified airflowoffset be removed as for example in step 27. The above arrangementtherefore provides a possible backup measure to the present invention.It should however be noted that the present control method can readilyoperate without such a back-up arrangement.

When the engine is operating in running mode, a simpler control strategyis used as shown in FIG. 4. At step 30, the time since the lastvariation of the adaption value while in the running mode or the timesince the entry to the running mode is compared against a specifiedtime. If this time is greater than the specified time, then the adaptionvalue is decremented by a specified amount in step 32. However, if theabove time is less than the specified time, then the adaption valueremains unchanged as shown at step 31.

The adaption value obtained at any of the above operating modes of theengine are used in determination of the purge solenoid control signal 9as shown in steps 3 and 4 of FIG. 1.

When various incrementations and decrementations to the adaption valueare made, a change in the adaption value of 0.1 is typical, but, ofcourse, may be varied.

What is claimed is:
 1. A method for controlling delivery of fuel from avapour collection device to at least one combustion chamber of aninternal combustion engine to thereby purge fuel that has accumulated inthe vapour collection device by means of a purge flow passing from thevapour collection device to the engine, the purge flow rate being variedby means of a flow control valve located between the vapour collectiondevice and the engine, the meth comprising the following steps,determining minimum valve signal value and a maximum valve signal valueas a function engine load and engine speed to thereby respectivelydefine a minimum and maximum extent of valve signal values forcontrolling opening of the valve, selecting either one of the minimumvalve signal value, the maximum valve signal value and an interpolationvalue that is between the maximum and minimum value signal values inwhich such selecting is performed in dependence on the engine operatingconditions to optimise an amount of fuel purged from the vapourcollection device, wherein the amount of interpolation between therespective minimum and maximum valve signal values is provided anadaptation value for obtaining an intermediate valve signal valuebetween said minimum and maximum signal values, the method furtherincluding determining an adaptation value as a function of the coolanttemperature of the engine, wherein the engine is operated under closedloop speed control at idle operation of the engine and is operated underopen loop control under other engine operating conditions.
 2. A methodaccording to claim 1 further including periodically varying the adaptionvalue with changing operating conditions of the engine.
 3. A methodaccording to claim 1 including determining an initial adaption valueupon first starting up the engine, such that the initial adaption valueis relatively low if the engine coolant temperature is relatively highto thereby restrict a purge air flow to the engine and such that theinitial adaption value is relatively high if the engine coolanttemperature is relatively low to thereby provide for a greater purge airflow to the engine.
 4. A method according to claim 1 when the engine isat idle, further including varying an adaption value by comparing theidle fueling level to the engine to a specified target fuelling level,incrementing the adaption value by a specified amount if the idlefuelling level is above the target fuelling level, and decrementing theadaption value by a specified amount if the idle fuelling level is belowthe target fuelling level.
 5. A method according to claim 1 furtherincluding adding a specified air flow offset when the adaption value isdecremented and removing a specified air flow offset when the adaptionvalue is incremented.
 6. A method according to claim 4 further includingcomparing the idle fuelling level to a specified limit value if the idlefuelling level is less than the specified target fuelling level, andsetting the adaption value to zero if the idle fuelling level is lessthan the limit value.
 7. A method according to claim 6 further includingadding a specified air flow offset if said adaption value is set tozero.
 8. A method according to claim 6 further including varying theadaption value following a specified period after a last variation ofthe adaption value or since the entry of the engine into idle operation.9. A method according to claim 1 when the engine is operating under openloop control, further including periodically decrementing the adaptionvalue by a preset amount.
 10. A method according to claim 1 wherein thefuel is purged into an intake manifold of the engine.
 11. An enginecontrol unit for, in use, controlling delivery of fuel from a vapourcollection device to at least one combustion chamber of an internalcombustion engine to thereby purge fuel that has accumulated in thevapour collection device by means of a purge flow passing from thevapour collection device to the engine, the purge flow rate being variedby means of a flow control valve located between the vapour collectiondevice and the engine, the engine control unit adapted to operateaccording to a method comprising the steps of determining a minimumvalve signal value and a maximum valve signal value as a function ofengine load and engine speed to thereby respectively define the minimumand maximum extent of valve signal values for controlling opening of thevalve, said function being independent of exhaust-air/fuel ratiofeedback control; and selecting either one of the minimum valve signalvalue, maximum valve signal value, and an interpolation between themaximum and minimum valve signal values in which said selecting isperformed in dependence on engine operating conditions so as to optimisean amount of fuel purged from the vapour collection device, wherein theamount of interpolation between the respective minimum and maximum valvesignal values is provided by an adaption value for obtaining anintermediate valve signal value between said minimum and maximum valvesignal values the method further including determining the adaptionvalue as a function of a coolant temperature of the engine, includingdetermining an initial adaption value upon first starting up the engine,such that the initial adaption value is relatively low if the enginecoolant temperature is relatively high to thereby restrict a purge airflow to the engine and such that the initial adaption value isrelatively high if the engine coolant temperature is relatively low tothereby provide for a greater purge air flow to the engine.
 12. Theengine control unit according to claim 11 further including periodicallyvarying the adaption value with changing operating conditions of theengine.
 13. The engine control unit according to claim 11 wherein theengine is operated under closed loop speed control at idle operation ofthe engine and is operated under open loop control under other engineoperating conditions.
 14. The engine control unit according to claim 13when the engine is at idle, further including varying the adaption valueby comparing an idle fueling level to the engine to a specified targetfueling level, incrementing the adaption value by a specified amount ifthe idle fueling level is above the target fueling level, anddecrementing the adaption value by a specified amount if the idlefueling level is below the target fueling level.
 15. The engine controlunit according to claim 11 further including adding a specified air flowoffset en the adaption value is decremented and removing a specified airflow offset when the adaption value is incremented.
 16. The enginecontrol unit according to claim 14 further including comparing the idlefueling level to a specified limit value if the idle fueling level isless than the specified target fueling level, and setting the adaptionvalue to zero if the idle fueling level is less than the limit value.17. The engine control unit according to claim 16 further includingadding a specified air flow offset if said adaption value is set tozero.
 18. The engine control unit according to claim 14 furtherincluding varying the adaption value following a specified period aftera last variation of the adaption value or since the entry of the engineinto idle operation.
 19. The engine control unit according to claim 13when the engine is operating under open loop control, further includingperiodically decrementing the adaption value by a preset amount.
 20. Theengine control unit according to claim 11 wherein the fuel is purgedinto the intake manifold of the engine.
 21. An internal combustionengine comprising at least one combustion chamber and adapted to, inuse, deliver fuel from a vapour collection device to said at least onecombustion chamber to thereby purge fuel that has accumulated in thevapour collection device by means of a purge flow passing from thevapour collection device to the engine, the purge flow rate being variedby means of a flow control valve located between the vapour collectiondevice and the engine, the engine adapted to operate according to amethod comprising the steps of determining a minimum valve signal valueand a maximum valve signal value as a function of engine load and enginespeed to thereby respectively define the minimum and maximum extent ofvalve signal values for controlling opening of the valve, said functionbeing independent of exhaust-air/fuel ratio feedback control; andselecting either one of the minimum valve signal valve, maximum valvesignal value, and an interpolation between the maximum and minimum valvesignal values in which said selecting is performed in dependence onengine operating conditions so as to optimise an amount of fuel purgedfrom the vapour collection device, wherein the amount of interpolationbetween the respective minimum and maximum valve signal values isprovided by an adaption value for obtaining an intermediate valve signalvalue between said minimum and maximum valve signal values, the methodfurther including determining the adaption value as a function of acoolant temperature of the engine, wherein the engine is operated underclosed loop speed control at idle operation of the engine and isoperated under open loop control under other engine operatingconditions.
 22. The engine according to claim 21 further includingperiodically varying the adaption value with changing operatingconditions of the engine.
 23. The engine according to claim 21 includingdetermining an initial adaption value upon first starting up the engine,such that the initial adaption value is relatively low if the enginecoolant temperature is relatively high to thereby restrict a purge airflow to the engine and such that the initial adaption value isrelatively high if the engine coolant temperature is relatively low tothereby provide for a greater purge air flow to the engine.
 24. Theengine according to claim 21 when the engine is at idle, furtherincluding varying the adaption value by comparing an idle fueling levelto the engine to a specified target fueling level, incrementing theadaption value by a specified amount if the idle fueling level is abovethe target fueling level, and decrementing the adaption value by aspecified amount if the idle fueling level is below the target fuelinglevel.
 25. The engine according to claim 21 further including adding aspecified air flow offset when the adaption value is decremented andremoving a specified air flow offset when the adaption value isincremented.
 26. The engine according to claim 24 further includingcomparing the idle fueling level to a specified limit value if the idlefueling level is less than the specified target fueling level, andsetting the adaption value to zero if the idle fueling level is lessthan the limit value.
 27. The engine according to claim 26 furtherincluding adding a specified air flow offset if said adaption value isset to zero.
 28. The engine according to claim 21 further includingvarying the adaption value following a specified period after a lastvariation of the adaption value or since the entry of the engine intoidle operation.
 29. The engine according to claim 21 when the engine isoperating under open loop control, further including periodicallydecrementing the adaption value by a preset amount.
 30. The engineaccording to claim 21 wherein the fuel is purged into the intakemanifold of the engine.